I recently noticed that iTunes now allows to swap out the "Song" name field for "Work Name", "Movement" & "Name" fields to better display classical music titles.
But these tags don't show up in MP3tag, does anyone know if these new fields actually tag the mp3 or are they just saved in iTunes?
I think that iTunes can be forced to write database information to files with the iTunes function to modify the tag version. It is possible to change the tag version from 2.3 to 2.3 (so that you do not actually change anything but the information is written anyway).
You could try this and see if there are any fields visible in MP3tag.